We studied some physiological indices to evaluate drivers' mental workload when they use driver assistance systems. In previous studies, Tissue Blood Volume (TBV) on the tip of nose and Skin Conductance Level (SCL) at palm showed good correlation with mental workload in a driving simulator experiment. In this study, these indices were investigated in on-road driving. Fifteen subjects were asked to maintain the distance to a forward vehicle that varied its own speed as an experimental task. The results showed that TBV was correlated with changes of the mental workload. Therefore TBV would be also a useful index for measuring drivers' mental workload in on-road driving.
